DE 10 2005 048 223 A1 discloses a paint coating system wherein a paint reservoir is connected, via an insulating cylinder, to a paint dosing device arranged upstream, wherein the paint dosing device doses the paint to be applied in controlled manner and feeds the paint to a rotary atomizer or other application equipment. The insulating cylinder enables electrical insulation of the paint dosing device in relation to the paint reservoir. This is advantageous since, with an electrostatic paint application method, the paint dosing device is usually at a high potential, whilst the paint reservoir, which is electrically insulated from the paint dosing device, is at earth potential and can therefore be refilled during the coating process. For electrical insulation, the paint residues located in the insulating cylinder are removed from the inner wall of the insulating cylinder by a scraping piston, in order to create the requisite electric strength. The scraping piston is mounted on one end of a piston rod which is displaceably mounted in a piston rod guide and supports a pneumatic piston at the other end thereof, which can be pneumatically driven in a pneumatic cylinder in order to displace the scraping piston in the insulating cylinder.
A problem with this known insulating cylinder is the fact that, in the region between the scraping piston and the piston rod guide, the piston rod is exposed to the paint to be applied, so that the paint adhering to the outer surface of the piston rod is able to penetrate through the piston rod guide into the pneumatic cylinder where the paint can then stick to and, in the worst case, block the pneumatic cylinder. In practice, the result is initially a slowing-down of the movement speed of the pneumatic cylinder and thus leads to cycle time problems. Eventually, the pneumatic cylinder can become entirely blocked, leading to cessation of production in the paint shop. A particular danger herein results from relatively long stoppage times (e.g. at night or over a weekend), when components become stuck due to hardening paint residues and then fail when production is restarted.
A further problem with the above-described known insulating cylinder is the unsatisfactory service life of the piston rod seal during ‘dry operation’ of the piston rod, during which small quantities of paint can dry on the piston rod, and this leads to increased abrasiveness and to premature failure of the piston rod seal.
With regard to the prior art, reference is also made to U.S. Pat. No. 4,863,101 A, DE 692 03 299 T2, U.S. Pat. No. 5,964,408 A, U.S. Pat. No. 4,469,251 A, U.S. Pat. No. 4,266,468 A, U.S. Pat. No. 4,126,321 A, DE 10 2005 060 959 A1, DE 692 28 249 T2, DE 195 24 853 C2 and U.S. Pat. No. 5,458,927 A. However, none of these citations describes a coating material supply device with an insulating cylinder comprising a piston rod seal which ensures satisfactory leak prevention.
Accordingly, there is a need for an improved insulating cylinder.